Rotor damper

ABSTRACT

A rotor stage ( 100 ) of a gas turbine engine ( 10 ) comprises a platform ( 120 ) from which rotor blades extend. The platform is provided with a circumferentially extending damper ring ( 200 ), the damper ring having an engagement surface ( 210 ) that engages with the platform. The damper ring has a cross-sectional shape perpendicular to the circumferential direction that has a depth (a) in the radial direction, and a neutral bending axis ( 250 ) that is spaced (b) from the engagement surface by more than half of the depth. Increasing the spacing between the engagement surface and the neutral bending axis increases the amount of slip at the interface between the damper ring and the platform, thereby increasing the amount of frictional damping.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromBritish Patent Application Number 1506195.5 filed 13 Apr. 2015, theentire contents of which are incorporated by reference.

BACKGROUND

1. Field of the Disclosure

The present disclosure concerns a damper for a rotating part of a gasturbine engine.

2. Description of the Related Art

A gas turbine engine comprises various stages of rotor blades whichrotate in use. Typically, a gas turbine engine would have at least onecompressor rotor stage, and at least one turbine rotor stage.

There are a number of ways in which the blades of a rotor stage may beattached to the engine. Generally, the blades attach to a rotatingcomponent, such as a disc, that is linked to a rotating shaft.Conventionally, blades have been inserted and locked into slots formedin such discs.

Integral bladed disc rotors, also referred to as blisks (or bliscs) havealso been proposed. Such blisks may be, for example, machined from asolid component, or may be manufactured by friction welding (for examplelinear friction welding) of the blades to the rim of the disc rotor.

Blisks have a number of advantages when compared with more traditionalbladed disc rotor assemblies. For example, blisks are generally lighterthan equivalent bladed disc assemblies in which the blades are insertedand locked into slots in the disc because traditional blade to discmounting features, such as dovetail rim slots, blade roots, and lockingfeatures are no longer required. Blisks are therefore increasingly usedin modern gas turbine engines, for example as part of the compressorsection (including the fan of a turbofan engine).

Typically blisks are designed where possible to avoid vibrationresponses from, for example, resonance and flutter, which may bedistortion driven. However, blisks lack inherent damping when comparedto conventional bladed disc assemblies and resonances and flutter cannotalways be avoided.

Additionally, the outer surface or rim of the blisk disc portiontypically forms the inner annulus for working fluid in the gas turbineengine, such as at the compressor inlet. Thus the requirement for theinner annulus position fixes the blisk outer rim radius from the enginecentre line thereby determining the basic size/shape of the discportion. Accordingly, it may not be possible to design a blisk thatavoids all forced vibration responses within such constraints.

OBJECTS AND SUMMARY

Accordingly, it is desirable to be able to provide efficient and/oreffective damping to a rotor stage, for example to a bladed disc, orblisk.

According to an aspect, there is provided a rotor stage of a gas turbineengine comprising a plurality of blades extending from a generallycircumferentially extending platform. A circumferentially extendingdamper ring is provided on the platform. The damper ring has anengagement surface that engages with the platform. The damper ring has across-sectional shape perpendicular to the circumferential direction.The cross-sectional shape has a depth in the radial direction, and aneutral bending axis that is spaced from the engagement surface by morethan half of the depth.

The rotor stage may comprise a disc. The plurality of blades may be saidto be mounted to the disc. Such a disc may be of any suitable form, forexample it may be a solid disc or an annular ring.

The damper ring may be provided to any desired and/or suitable surfaceof the platform, for example a damper ring may be provided to a radiallyinner surface and/or a radially outer surface thereof.

In arrangements in which the damper ring is provided on the radiallyinner surface of the platform, the damper ring may be said to have aneutral bending axis that is closer to its radially inner extent than toits radially outer extent.

In arrangements in which the damper ring is provided on the radiallyouter surface of the platform, the damper ring may be said to have aneutral bending axis that is closer to its radially outer extent than toits radially inner extent.

In arrangements in which the damper ring is provided on the radiallyinner surface, the radially outer extent of the damper ring may bedefined by the engagement surface (which may be referred to as a contactsurface), which may be said to engage with (or be in contact with) theradially inner surface of the platform.

In arrangements in which the damper ring is provided on the radiallyouter surface, the radially inner extent of the damper ring may bedefined by the engagement surface (which may be referred to as a contactsurface), which may be said to engage with (or be in contact with) theradially outer surface of the platform.

The engagement surface may, for example, form a cylinder, or afrusto-conical shape.

According to an aspect, there is provided a rotor stage of a gas turbineengine a plurality of blades extending from a generallycircumferentially extending platform. The platform has a radially innersurface. A circumferentially extending damper ring is provided on theradially inner surface of the platform. The damper ring has across-sectional shape perpendicular to the circumferential directionthat has a neutral bending axis that is closer to its radially innerextent than to its radially outer extent.

According to an aspect, there is provided a method of damping vibrationsin a rotor stage of a gas turbine engine, the rotor stage being asdescribed and/or claimed herein. The vibration may comprise a travellingwave passing circumferentially around the circumferentially extendingplatform. Such a travelling wave may result in diametral modeexcitation, for example of the platform. The damping may be frictionaldamping generated through slip (which may be circumferential slip)between circumferentially extending damper ring and the platform (forexample the radially inner and/or radially outer surface of theplatform) caused by the travelling wave.

In arrangements in which the damper ring is provided to a radially innersurface of the platform, the damper ring may be said to have a neutralbending surface that is closer to its radially inner extent than to itsradially outer extent.

Providing a damper ring as described and/or claimed herein may provideimproved damping. The further the neutral axis of the damper ring isfrom the engagement surface of the damper ring the greater the slip(which may be referred to as the relative movement) at the interfacebetween the damper ring and the platform that results from a travelingwave passing around the rotor stage (for example around the platform).This increased slip may lead to increased frictional energy beingdissipated at the interface between the damper ring and the platform.This increased energy dissipation may lead to improved damping.

Any suitable shape may be used for the damper ring, for example for thecross-sectional shape of the damping ring. For example, thecross-sectional shape of the damper ring may be a T-shape, for examplewith the base of the T (where the base is the horizontal part of the “T”as presented on this page) provided at an opposite side to theengagement surface (for example an inverted T-shape, with the base ofthe T-shape provided at a radially inner extent, for arrangements inwhich the damper ring is provided to a radially inner surface of theplatform).

The cross-sectional shape of the damper ring may comprise a portion thatwidens with increasing distance from the engagement surface (for examplewith decreasing radial distance for arrangements in which the damperring is provided to the radially inner platform). For example, amajority portion of the damper ring may widen with increasing distancefrom the engagement surface.

By way of further non-limitative example, the cross-sectional shape ofthe damper ring may comprise a trapezium.

The damper ring may have a generally annular shape. The damper ring mayextend around all, or a majority, of the circumference of the rotorstage.

The cross-sectional shape of the damper ring may have a neutral axisthat is spaced from the engagement surface by at least two thirds (forexample three quarters, four fifths, five sixths or more than fivesixths) of the cross-sectional depth. For arrangements in which thedamper ring is provided to a radially inner surface of the platform, thecross-sectional shape of the damper ring may have a neutral axis that isat least twice as far—for example three times, four times, five times ormore than five times—from its radially outer extent than from itsradially inner extent.

The platform may be provided with a slot. The damper ring may beretained by such a slot. The damper ring may be said to sit in and/or belocated by and/or at least partly located in such a slot. Such a slotmay be said to engage with the radially outer extent (for exampleradially outer surface) of the damper ring, where the damper ring isprovided to a radially inner surface of the platform.

The damper ring may be manufactured using any suitable material. Forexample, the damper ring may be manufactured using a single materialand/or may be said to be homogeneous. The damper ring may comprise two(or more than two) different materials.

The damper ring may have a body portion and an engagement portion. Theengagement portion may comprise the engagement surface that is incontact with the platform. Regardless of the material of the damper ring(for example whether it is manufactured using one, two, or more than twomaterials), the engagement surface may be the surface that slipsrelative to the platform during excitation (or vibration) of theplatform. In arrangements in which the damper ring comprises a bodyportion and an engagement portion, the engagement portion may bemanufactured using a first material, and the body portion may bemanufactured using a second material. In such an arrangement, and purelyby way of example only, the first material may be metal and/or thesecond material may be a composite, such as a fibre reinforced and/orpolymer matrix composite, such as carbon fibre. In such an arrangement,the body portion and the engagement portion may, for example, be bondedtogether.

According to an aspect, there is provided a rotor stage of a gas turbineengine comprising a plurality of blades extending from a generallycircumferentially extending platform from which the blades extend. Therotor stage comprises a circumferentially extending damper ring isprovided on a circumferentially extending surface of the platform.According to this aspect, the damper ring has a body portion and anengagement portion. The engagement portion has an engagement surfacethat is in contact with the platform. The engagement portion ismanufactured using a first material, and the body portion ismanufactured using a second material. According to this aspect, thedamper ring may have any suitable cross-sectional shape, including thosedescribed and/or claimed herein.

In any aspect and/or arrangements, the rotor stage may be a blisk. Theplurality of blades may be provided integrally with a disc as a unitarypart. In arrangements in which the rotor stage is a blisk, thecircumferentially extending platform may be a blisk rim.

In any arrangement, a lubricant, such as a dry film lubricant, may beprovided between the engagement surface of the damper and the platform.Such a lubricant may assist in providing a particularly consistentcoefficient of friction at the engagement surface, for example duringuse and/or over time.

Any feature described and/or claimed herein, for example in relation toany one of the above features, may be applied/used singly or incombination with any other feature described and/or claimed herein,except where mutually exclusive.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be described by way of example only, with reference tothe Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine in accordancewith an example of the present disclosure;

FIG. 2 is a schematic view of a part of a rotor stage of a gas turbineengine, including a damper ring, in accordance with an example of thepresent disclosure;

FIG. 3 is a schematic view of a part of a rotor stage of a gas turbineengine, including a damper ring, in accordance with an example of thepresent disclosure;

FIG. 4 is a schematic showing slip at an interface between a damper ringand a platform;

FIG. 5 is a close-up view of a part of the FIG. 4 schematic;

FIG. 6 is a schematic cross-sectional view of a damper ring inaccordance with an example of the present disclosure;

FIG. 7 is a schematic cross-sectional view of a damper ring inaccordance with an example of the present disclosure;

FIG. 8 is a schematic cross-sectional view of a damper ring inaccordance with an example of the present disclosure;

FIG. 9 is a schematic cross-sectional view of a damper ring inaccordance with an example of the present disclosure; and

FIG. 10 is a schematic cross-sectional view of a damper ring inaccordance with an example of the present disclosure;

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to FIG. 1, a gas turbine engine is generally indicated at10, having a principal and rotational axis 11. The engine 10 comprises,in axial flow series, an air intake 12, a propulsive fan 13, anintermediate pressure compressor 14, a high-pressure compressor 15,combustion equipment 16, a high-pressure turbine 17, and intermediatepressure turbine 18, a low-pressure turbine 19 and an exhaust nozzle 20.A nacelle 21 generally surrounds the engine 10 and defines both theintake 12 and the exhaust nozzle 20.

The gas turbine engine 10 works in the conventional manner so that airentering the intake 12 is accelerated by the fan 13 to produce two airflows: a first air flow into the intermediate pressure compressor 14 anda second air flow which passes through a bypass duct 22 to providepropulsive thrust. The intermediate pressure compressor 14 compressesthe air flow directed into it before delivering that air to the highpressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 isdirected into the combustion equipment 16 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 17, 18, 19 before being exhausted through thenozzle 20 to provide additional propulsive thrust. The high 17,intermediate 18 and low 19 pressure turbines drive respectively the highpressure compressor 15, intermediate pressure compressor 14 and fan 13,each by suitable interconnecting shaft.

Each of the high 17, intermediate 18 and low 19 pressure turbines andeach of the fan 13, intermediate pressure compressor 14 and highpressure compressor 15 comprises at least one rotor stage havingmultiple blades (or aerofoils) that rotate in use. One or more rotorstage may be, for example, a disc with slots (which may be referred toas dovetail slots or fir-tree slots) for receiving the blade roots. Oneor more rotor stages may have the blades formed integrally with thesupporting disc or ring structure, and may be referred to as blisks orblings. In such arrangements, the blades may be permanently attached tothe supporting disc/ring, for example using friction welding, such aslinear friction welding.

FIG. 2 shows a schematic side view of a part of a rotor stage 100,including a platform 120, a disc 140, a blade 160, and a damper ring200. The platform 120, disc 140 and blade 160 may all be integral, andmay be referred to collectively as a blisk 180. The rotor stage 100 maybe any one of the rotor stages of the gas turbine engine 10 shown inFIG. 1, such as (by way of non-limitative example) the fan 13 and/or anyone or more stages of one or more of the high 17, intermediate 18 andlow 19 pressure turbines and/or the high pressure compressor 15 orintermediate pressure compressor 14.

In the FIG. 2 example, the damper ring 200 is provided to the lower (orradially inner) surface 122 of the platform 120. In other arrangementsthe damper ring 200 may engage with another part of the platform 120such as, by way of example, an upper (or radially outer) surface 124 ofthe platform 120.

The damper ring 200 has an engagement surface 210 that engages with theplatform 120. The engagement surface 210 may be at a radially outerextent (and/or may be said to be and/or define the radially outersurface of the damper ring 120), as in the FIG. 2 arrangement. Theengagement surface 210 of any arrangement may be said to extendcircumferentially and/or may be at least a part of a cylinder orfrusto-cone.

The damper ring 200 is shown in cross-section normal to thecircumferential direction in FIG. 2. The cross-sectional shape of thedamper ring 200 is such that its neutral bending axis is spaced from theengagement surface 210 by more than half of the depth of the damperring, this depth being indicated by reference ‘a’ in the FIG. 2 example.Any suitable shape of damper ring 200 may be used to achieve this.Purely by way of example, the cross-sectional shape of the damper ring200 shown in FIG. 2 may be described as a trapezium, or trapezoid. Thenarrower side of the trapezium is at the engagement surface 210 (at aradially outer extent in the FIG. 2 example). The wider side of thetrapezium is opposite to the engagement surface 210 (at a radially innerextent in the FIG. 2 example).

The example shown in FIG. 3 is similar to that shown in, and describedin relation to, FIG. 2. Like features in FIGS. 2 and 3 are indicatedwith like reference numerals. Unlike the FIG. 2 arrangement, the FIG. 3arrangement has a slot 130 provided in the platform 120. The damper ring200 is provided (and/or located and/or secured) in the slot 130. Theslot 130 may be said to receive the damper ring 200. The slot 130 may besaid to extend circumferentially.

In the FIG. 3 arrangement the slot 130 is provided to the radially innersurface 122 of the platform 120, although in other arrangements the slot130 may be provided to the radially outer surface 124 of the platform120. The cross-sectional shape (perpendicular to the circumferentialdirection) of the slot 130 corresponds to the cross-sectional shape of aportion of the damper ring 200 that is adjacent the engagement surface210. In the FIG. 3 arrangement this corresponds to a radially outerportion of the damper ring 200.

Any of the damper rings 200 described and/or claimed herein may functionthrough frictional damping. Such frictional damping occurs as a resultof relative movement at the interface between the damper ring 200 andthe surface with which it is engaged. For example, the relative movementmay be between the engagement surface 210 of the damper ring 200 and thesurface of the platform 120 (for example the radially inner 122 orradially outer 124 surface) with which it is engaged. This relativemovement and frictional damping mechanism is described in greater detailin relation to FIGS. 4 and 5.

FIG. 4 shows a schematic of part of a platform 120 and damper ring 200in a deformed state, for example due to vibration. The engagementsurface 210 is shown in the deformed state in FIG. 4, and the line 300in FIG. 4 represents the original, undeformed, interface between theplatform 120 and the damper ring 200.

The FIG. 4 view is a purely schematic representation perpendicular tothe rotational axis 11 of the rotor stage 100. The view may be describedas being in the radial-circumferential plane. Thus, although theundeformed interface (or engagement surface) 300 is shown as a straightline in FIG. 4 for ease of representation and explanation, it will beappreciated that the actual undeformed interface would be at least asegment of a circle. Accordingly, the deflection y represents thedeflection from the original circular shape, and the dimension xrepresents the distance around the circumference of the interface.

FIG. 5 shows a close up version of the region labelled “P” in FIG. 4.This region P is the region of maximum slip at the interface between theplatform 120 and the damper ring 200, i.e. maximum slip at theengagement surface 210.

The neutral bending axis of the damper ring 200 is shown in FIGS. 4 and5 by dashed line 250, and the neutral bending axis of the platform 120is shown by dashed line 150. The neutral bending axis 150, 250 shown inthe figures may be part of a neutral bending surface for the respectivecomponent. The neutral bending axis/surface 150, 250 of a component 120,200 may be defined as the axis/surface that does not experience anystress and/or strain (for example in the circumferential direction) asthe component deforms, for example due to vibration. The shearstress/strain may be said to be at maximum on the neutral axis/surface.The position of these neutral bending axes is shown purely schematicallyin FIGS. 4 and 5, for the purpose of explaining the frictional dampingmechanism.

The thickness, or depth, of the damper ring 200 is shown by reference a.The distance of the neutral bending axis 250 of the damper ring 200 fromthe engagement surface 210 is labelled b in FIGS. 4 and 5. The distanceof the neutral bending axis 150 of the platform 120 from the engagementsurface 210 is labelled c in FIGS. 4 and 5.

Frictional damping may occur due to relative movement between the damperring 200 and the platform 120 at the engagement surface 210. In otherwords, as the platform 120 and the damper ring 200 vibrate, thedeflection y at a given position x around the circumference changes, andmay be said to oscillate about the centreline, or undeformed line, 300.This oscillation leads to varying relative sip at the interface 210, andthus the generation of frictional energy that acts to damp thevibration.

FIG. 4 represents half of a wavelength of a sinusoidal vibration. If theradius of the undeformed interface 300 is given by R, and the totalnumber of waves (or “nodal diameters”) around the circumference is givenby N, then it follows that the deflection of the engagement surface 210at a given location x around the circumference is given by:

$y = {Y\; {\cos \left( {N\; \frac{x}{R}} \right)}}$

where Y is the maximum deflection.

Differentiating this to find the slope gives:

$\frac{y}{x} = {{{- \frac{NY}{R}}{\sin \left( \frac{Nx}{R} \right)}} = {\tan (\alpha)}}$

where α is the angle from the undeformed shape.

This has a maximum magnitude at (for example)×=πR/2N of:

${\tan (\alpha)} = \frac{NY}{R}$

This value is in the region P shown in detail in FIG. 5, from which itis clear that:

${\tan (\alpha)} = {\left. \frac{s}{d}\Rightarrow\frac{s}{d} \right. = \left. \frac{NY}{R}\Rightarrow\begin{matrix}{s = \frac{NYd}{R}}\end{matrix} \right.}$

where s is the maximum slip, and d is the separation between the neutralaxes 150, 250 of the platform 120 and damper ring 200.

Accordingly, it is clear that the maximum slip value is a function of(in this model proportional to) the separation d between the neutralaxes 150, 250 of the platform 120 and the damper ring 200. Because it isthe slip s that generates the friction to dissipate the energy that inturn provides the damping, it is advantageous to maximise (or increase)this value. This may be achieved by increasing the value of d, i.e.increasing the separation between the neutral axes 150, 250 of theplatform 120 and the damper ring 200.

More effective damping may be realized by increasing the distance b ofthe neutral axis 250 of the damper ring 200 from the engagement surface210. Accordingly, this may be achieved by setting the distance b to beat least half of the depth a of the damper ring. This may be achievedthrough the use of a great many different cross-sectional shapes(perpendicular to the circumferential direction) for the damper ring200.

Examples of different cross-sectional shapes of the damper ring 200 areshown in FIGS. 6 and 7. It will be understood that these shapes aremerely examples of many different shapes that could be used to achieveimproved damping.

FIG. 6 shows a T-shaped damper ring 200. The damper ring 200 may bedescribed as having an inverted T-shape. The T-shape is thecross-sectional shape as viewed perpendicular to the circumferentialdirection. This T-shape results in a neutral bending axis 250 that iscloser to the radially inner extent 260 of the damper ring 200 than tothe radially outer extent 210. The radially outer extent 210 of thedamper ring 200 shown in FIG. 6 corresponds to the engagement surface210. The distance b of the neutral bending axis 250 from the engagementsurface 210 is more than half of the depth a of the damper ring 200.

Whilst the damper ring 200 in FIG. 6 is shown as being provided to theradially inner surface 122 of the platform 120, a T-shaped damper ring200 may instead be provided to the radially outer surface 124 of theplatform 120. In that case, the “T” shape would be inverted from thatshown in FIG. 6, such that the distance b of the neutral bending axis250 from the engagement surface 210 remains more than half of the deptha of the damper ring 200, with the engagement surface 210 engaging withthe upper surface 124.

FIG. 7 shows an alternative arrangement of damper ring 200, in which thecross-sectional shape is a trapezium. Such a damper ring 200 may besimilar to that shown in FIG. 2 and/or 3. As with the damper ring shownin FIG. 6, the damper ring 200 shown in FIG. 7 has a neutral bendingaxis 250 that is spaced b from the engagement surface 210 by more thanhalf of the depth a of the damper ring 200. Again, the damper ring 200may be provided to either the radially inner surface 122 (as shown) orto the radially outer surface 124 of the platform 120 (in which case itwould be inverted relative to the FIG. 7 arrangement).

FIGS. 8 and 9 show further exemplary arrangements of rotor stages 100,each comprising a damper ring 200 and platform 120. Both arrangementsinclude a slot 130 in the lower (or radially inner) surface 122 of theplatform 120. The damper ring 200 is retained by the slot 130. The shapeof the slot 130 (for example the cross-sectional shape of the slot 130)may be said to correspond to an engagement portion (which in theillustrated examples corresponds to a radially outer portion) of thedamper ring 200. In the examples of both FIGS. 8 and 9, the neutralbending axis 250 is further than 50% of the depth a of the damper ringaway from the engagement surface (or radially outer surface, in theillustrated examples) 210 of the damper ring 200.

The damper ring 200 of the FIG. 8 example comprises a body portion 202and an engagement portion 204. The body portion 202 and the engagementportion 204 are manufactured using different materials in the FIG. 8example. For example, the body portion 202 may be manufactured using acomposite material, such as a polymer matrix composite and/or afibre-reinforced composite, such as carbon fibre. The engagement portion204 may be manufactured using a material that is more wear-resistantthan that of the body portion 202 such as, for example, a metal, forexample titanium.

The FIG. 9 example is broadly similar to the FIG. 6 example, other thanin that that platform 120 of the FIG. 9 example is provided with theslot 130.

For arrangements in which the damper ring 200 is retained in a slot 130,the slot 130 may be said to have a base surface 132 and side surfaces134. The base surface 132 may extend generally perpendicularly to (forexample have a major component perpendicular to) the radial direction.The side surfaces 134 may extend generally perpendicularly to (forexample may have a major component perpendicular to) the axial (orrotational) direction. The damper ring 200 may, optionally, engage withboth the base surface 132 and the side surfaces 134. In that case, theengagement surface 210 of the damper ring may be defined as being thatsurface 210 which engages with the base surface 132 (or at least isclosest to the base surface 132 of the slot 130).

FIG. 10 shows a further exemplary arrangement of rotor stage 100comprising a platform 120 and a damper ring 200. The damper ring 200 ofFIG. 10 sits inside a slot 130. The damper ring 200 of FIG. 10 has abody portion 202 and an engagement portion 204. The body portion 202 andthe engagement portion 204 are manufactured using different materials inthe FIG. 10 example. For example, the body portion 202 may bemanufactured using a composite material, such as a polymer matrixcomposite and/or a fibre-reinforced composite, such as carbon fibre. Theengagement portion 204 may be manufactured using a material that is morewear-resistant than that of the body portion 202 such as, for example, ametal, for example titanium. In the FIG. 10 example, the damper ring 200is provided in a slot 130 on the underside (radially inner) surface 122of the platform 120. However, in other arrangements, platform may not beprovided with a slot 130 and/or the damper ring 200 may be provided tothe radially outer surface 124 of the platform 120.

It will be understood that the invention is not limited to thearrangements and/or examples above-described and various modificationsand improvements can be made without departing from the conceptsdescribed and/or claimed herein. Except where mutually exclusive, any ofthe features may be employed separately or in combination with any otherfeatures and the disclosure extends to and includes all combinations andsub-combinations of one or more features described and/or claimedherein.

We claim:
 1. A rotor stage of a gas turbine engine comprising aplurality of blades extending from a generally circumferentiallyextending platform, wherein: a circumferentially extending damper ringis provided on the platform, the damper ring having an engagementsurface that engages with the platform; and the damper ring has across-sectional shape perpendicular to the circumferential directionthat has a depth (a) in the radial direction, and a neutral bending axisthat is spaced (b) from the engagement surface by more than half of thedepth.
 2. A rotor stage of a gas turbine engine according to claim 1,wherein: the platform has a radially inner surface; the damper ring isprovided on the radially inner surface of the platform; and the damperring has a neutral bending axis that is closer to its radially innerextent than to its radially outer extent.
 3. A rotor stage of a gasturbine engine according to claim 1, wherein the cross-sectional shapeof the damper ring is a T-shape.
 4. A rotor stage of a gas turbineengine according to claim 1, wherein the cross-sectional shape of thedamper ring comprises a portion that widens with increasing distancefrom the engagement surface.
 5. A rotor stage of a gas turbine engineaccording to claim 1, wherein the damper ring has a generally annularshape.
 6. A rotor stage of a gas turbine engine according to claim 1,wherein the neutral axis of the cross-sectional shape of the damper ringis spaced from the engagement surface by at least two thirds of thecross-sectional depth.
 7. A rotor stage of a gas turbine engineaccording to claim 1, wherein the radially outer extent of the damperring is defined by the engagement surface that is in contact with theradially inner surface of the platform.
 8. A rotor stage of a gasturbine engine according to claim 1, wherein the platform is providedwith a slot, and the damper ring is retained by the slot.
 9. A rotorstage of a gas turbine engine according to claim 1, wherein the rotorstage is a blisk, with the plurality of blades provided integrally witha disc as a unitary part to form the blisk.
 10. A rotor stage of a gasturbine engine according to claim 9, wherein the circumferentiallyextending platform is a blisk rim.
 11. A rotor stage of a gas turbineengine according to claim 1, wherein the damper ring comprises twodifferent materials.
 12. A rotor stage of a gas turbine engine accordingto claim 11, wherein the damper ring has a body portion and anengagement portion, the engagement portion comprising the engagementsurface; and the engagement portion is manufactured using a firstmaterial, and the body portion is manufactured using a second material.13. A rotor stage according to claim 12, wherein: the first material ismetal.
 14. A rotor stage according to claim 12, wherein: the secondmaterial is a composite material.
 15. A rotor stage of a gas turbineengine comprising a plurality of blades extending from a generallycircumferentially extending platform, wherein: a circumferentiallyextending damper ring is provided on the platform, the damper ringhaving a body portion and an engagement portion, the engagement portionhaving an engagement surface that is in contact with the platform; andthe engagement portion is manufactured using a first material, and thebody portion is manufactured using a second material.
 16. A rotor stageaccording to claim 15, wherein: the first material is metal.
 17. A rotorstage according to claim 15, wherein: the second material is a compositematerial.
 18. A method of damping vibrations in a rotor stage of a gasturbine engine, wherein: the rotor stage is a rotor stage according toclaim 1; the vibration comprises a travelling wave passingcircumferentially around the circumferentially extending platform; andthe damping is frictional damping generated through slip (s) between theengagement surface of the circumferentially extending damper ring andthe platform caused by the travelling wave.